Field of the Invention
The present invention relates generally to the electric grid. More specifically, the present invention relates to reducing overvoltage and in-rush current in high power rectifier input filters used in medium-voltage grid applications.
Description of Related Art
In the generation, transmission, and distribution of conventional electrical power, power is transmitted over long distances, via the electric grid, at high alternating current (AC) voltages. In some cases, and for various reasons, these high AC voltages must be transmitted as high direct current (DC) voltages. A fundamental principle of DC transmission is using a rectifier to transform the AC voltages to DC voltages. High power voltage source, or current source, rectifiers are often used in this process.
As understood by those of skill in the art, high power voltage source or current source rectifiers are normally fed by a medium voltage grid supply. An inductor capacitor (LC) input filter is used to feed the medium voltage grid supply. The LC input filter also assists the commutation process of related power devices and the filtering out of line current harmonics typically associated with high power rectification.
More specifically, the LC filter is designed to achieve a desired level of attenuation of the switching harmonics and to avoid the amplification of residual harmonics during normal operation. However, for high power applications, the input filter for high power rectifiers is lightly damped to avoid excessive power losses.
Inherent power losses in the input filter and the supply transformer provide a minimum degree of damping. Such a low inherent damping factor of the input LC filter usually results in the excessive over-voltage stress across the input filter capacitors followed by a direct connection of the input filter to the medium-voltage grid supply.
The conventional pre-charge solution to this problem is to employ series-connected resistors to reduce both the over-voltage across the capacitors and the associated in-rush current. This solution, however, requires the use of bulky pre-charge resistors. These bulky pre-charge resistors not only degrade the overall system efficiency, but leave limited space for optimization of the pre-charge of the input filter.
Other conventional pre-charge solutions employ additional pre-charge circuit components such as the pre-charge resistor and contactors, resulting in increased system costs and diminish overall system efficiency. These undesirable consequences are especially applicable to high power AC-DC converters directly fed by a medium-voltage grid supply (e.g: 6 kV+). For worst case here, the in-rush current results in serve current distortions at the grid terminal if the pre-charge resistor is not heavy damped.
Other conventional solutions include use an AC side pre-charge instead of a DC side pre-charge. Use of an AC side pre-charge instead of a DC-side pre-charge, however, faces the challenge of having the filter capacitor assume a very small value. The small filter capacitor value creates a correspondingly small impedance formed the connection from each phase to the ground. Therefore, this approach also fails to eliminate the in-rush current and over-voltage.